Decouple AverageLagTracker and LatencyInfo.

Moving the event collection to LayerTreeHostImpl is an initial step
towards a new version of the AverageLag metrics using
PresentationFeedback times instead of GpuSwap times, which are available
through LayerTreeHostImpl.
Moving them from LatencyTracker is also interesting as it aggregates
events across all viz clients at the end of the GPU pipeline, which can
potentially mix up events from multiple sources.
Tracking events by viz client (through LayerTreeHostImpl) avoids it.

As AverageLagTracker should be dedicated to Telemetry, it is interesting
to remove all LatencyInfo preprocessing from it, as LatencyInfo stores
a lot of information that is irrelevant to AverageLagTracker.

This CL proposes to move the scroll events collection from LatencyTracker
to LayerTreeHostImpl. It also proposes an AverageLagTrackingManager,
an intermediary class for using AverageLagTracker that preprocesses
LatencyInfo objects so AverageLagTracker can be independent from it.

As the event collection is moved to LayerTreeHostImpl, the
AverageLagTrackingManager also serves to encapsulate any preprocessing
from LayerTreeHostImpl and AverageLagTrakcer.


Bug: 1079024, 989207
Change-Id: I2e73e244eee9565239a638f00dfbf19d5658eadb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2189633
Commit-Queue: João Victor Almeida de Aguiar <joalmei@microsoft.com>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#785975}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 7e5e837..ce648504 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -149,6 +149,10 @@
     "layers/video_layer_impl.h",
     "layers/viewport.cc",
     "layers/viewport.h",
+    "metrics/average_lag_tracker.cc",
+    "metrics/average_lag_tracker.h",
+    "metrics/average_lag_tracking_manager.cc",
+    "metrics/average_lag_tracking_manager.h",
     "metrics/begin_main_frame_metrics.cc",
     "metrics/begin_main_frame_metrics.h",
     "metrics/compositor_frame_reporter.cc",
@@ -660,6 +664,8 @@
     "layers/video_frame_provider_client_impl_unittest.cc",
     "layers/video_layer_impl_unittest.cc",
     "layers/viewport_unittest.cc",
+    "metrics/average_lag_tracker_unittest.cc",
+    "metrics/average_lag_tracking_manager_unittest.cc",
     "metrics/compositor_frame_reporter_unittest.cc",
     "metrics/compositor_frame_reporting_controller_unittest.cc",
     "metrics/compositor_timing_history_unittest.cc",